You've written the program and it doesn't work.
Check out the list of common programming errors
dbx. However, the commands and usage vary from vendor to vendor. Also, many vendors have a graphical debugging and performance system. The GNU debugger is called
Debuggers are especially useful for tracing segmentation faults and bus errors. They will pinpoint exactly where the program has stopped.
One can run the program under the debugger, or use the debugger to examine a core file after a program has crashed.
A tutorial on
dbx is for an IBM SP, but the basic ideas are similar
on other computers.